This invention relates to force-measurement devices using circularly polarized laser beams and more particularly to a biasing element for improving the operation of such devices.
The reference believed to be most relevant to the present invention is U.S. Pat. No. 3,786,681, issued to Kiehn, on Jan. 22, 1974, entitled "Electromagnetic Wave Modulation and Measurement System and Method." This patent discloses a preferred embodiment comprising a ring laser in which the laser beams have at least two different circular polarization modes. A modulator element comprising a quartz rod is positioned so that the laser beam travels the length of the rod. When a torque is applied to the rod, the two circular polarization modes change frequency differentially to provide a frequency difference proportional to the torque. In a gravimeter configuration, the torque is generated by suspending a mass on the end of an arm which is attached to one end of the quartz rod.
As noted in that patent, this force-measurement system is linear for forces above some level. Near the zero force level, where the two polarization modes have essentially the same frequency, there is believed to be a phase-locking effect which resists the change in frequency caused by the applied force. It is suggested in the patent that the discontinuity can be avoided by applying a bias force to the pressure-sensing element or by pre-stressing the element or by careful design of the coatings on the mirror and window surfaces making up the laser cavity. No specific method for applying the bias force or prestressing the modulating element is given, even though it is clear that any such arrangement adds to the desired signal and that it is essential that the bias level be absolutely constant or at least be known so that it may be separated from the desired signal.
When the apparatus described in the patent is used as an accelerometer for detecting horizontal acceleration, it is clear that the apparatus would tend to be biased at a zero beat frequency when there is no horizontal acceleration. When used, for example, in an aircraft to detect right and left acceleration, a device biased at zero beat frequency for zero acceleration would not be able to distinguish between right and left acceleration but would only give the amplitude of the acceleration. To provide the directional indication, a bias level at least as large as the maximum acceleration level to be detected must be provided, and it is important to know what that bias level is so that, for example, signals larger than the bias level could indicate a right turn acceleration while signals less than the bias level could indicate a left turn acceleration.
Accordingly, an object of the present invention is to provide a biasing element for a ring laser force-measurement system which provides a constant known level of bias.
A second object of the present invention is to provide a bias arrangement for a ring laser force-measurement system in which the bias level may be independently monitored without interference with the primary force-measurement system.
A bias element according to the present invention comprises a photoelastic element having a longitudinal axis aligned with and in the path of a laser beam in a force-measurement system. The biasing element is bonded at its ends to a stressing element with the biasing element and the stressing element pre-stressed prior to the bonding step. In a preferred form, the stressing element is made of the same material as the biasing element and a second ring laser is used to monitor the bias level by passing a laser beam through the stressing element.